Quote - ...the scene is showing hdri-25_color W.hdri in its settings, but I can't find that
file anywhere on any hard drive...

That one is 865k (750 x 375 pixels). The files are in scenes/global illumination folder.
The bigger .hdr files are in different folder (Presets/TEXTURES/HDRI TEXTURES). Those range arond 3000x1500 pixels. The hi-res Dosch.hdr in the presets folder is 6250 x 3125. Even the 3000 pixel are a bit blurry, if the render size is relatively large; somewhat over 800 pixel render size or so and they start to pixellate.

So, I'm thinking that using a 750 x 375 image for a background wrapped around a whole sphere would positively be really blurry and/or pixellated.

Friends don't let friends use booleans.